Central Columbia Plateau - Yakima River Basin NAWQA Study - Publications

Inputs of nitrogen and changes of nitrate concentrations in ground water underlying irrigated cropland in parts of eastern Washington

By James C. Ebbert, U.S. Geological Survey, Washington District

Abstract

Concentrations of nitrate in ground water underlying irrigated cropland in some areas of eastern Washington have increased by as much as 100 times since the early 1950s when those areas were opened to irrigation. Although few samples were collected prior to irrigation, most concentrations of nitrate in ground water in Columbia Basin Irrigation Project (CBIP) area, which includes parts of Grant, Adams, and Franklin Counties, were about 1 mg/L, or less, at that time. A recent (1994) summary of available data, most from the 1980s, made by the U.S. Geological Survey's National Water-Quality Assessment (NAWQA) program indicated that concentrations of nitrate in water from about 30 percent of wells sampled in the CBIP area equaled or exceeded the maximum contaminant level (MCL) for drinking water of 10 mg/L. The median concentration of nitrate in those samples was 5.4 mg/L. Estimated proportions of nitrogen inputs to the CBIP area are fertilizer, about 80 percent; livestock, about 15 percent; and other sources, about 5 percent. Although applied nitrogen fertilizers are the chief source of nitrogen, other sources, typically feedlots and dairies, also affect the distribution of nitrate in ground water. At one location, just west of the CBIP area, natural nitrate also may be a significant source of nitrate to ground water. Current temporal trends of concentrations of nitrate in ground water are difficult to determine on a regional basis because of the sparse distribution of wells with long-term monitoring data. Although trends vary locally, rates of increase generally are leveling off and in some areas, concentrations are steady.

CITATION: Ebbert, J.C., 1995, Inputs of nitrogen and changes of nitrate concentrations in ground water underlying irrigated cropland in parts of eastern Washington, in Nitrogen Environment and People Conference, Everett, Washington, February 1995, p. 52.